Exhaust gas from internal combustion engines such as automobile engines include hydrocarbons (hereunder also abbreviated as “HC”) and the like, and it is known that such substances can be removed by combustion catalysts. Also, with engine exhaust gas purification catalysts, they can be removed by so-called three-way catalysts for exhaust gas purification that allow oxidation of the carbon monoxide (CO) and hydrocarbons simultaneously with reduction of the nitrogen oxides (hereunder also abbreviated as NOx).
As a means of absorbing fluctuations in oxygen concentration of exhaust gas and increasing exhaust gas purification power, it is known to use a material with oxygen storage capacity (hereunder also abbreviated as “OSC”) that can store oxygen when the oxygen concentration of the exhaust gas is high and release oxygen when the oxygen concentration of the exhaust gas is low, as a support for the exhaust gas purification catalyst.
It is known that cerium oxides and cerium zirconium complex oxides, or their solid solutions, are excellent materials as OSC, and these are either combined with alumina supports, so that their chemical properties and those of the supports are utilized to promote purification of exhaust gas, or they may be used without such combination.
PTL 1 describes a ceria-zirconia-based complex oxide comprising a complex oxide of ceria and zirconia, a pyrochlore phase-type ordered array phase being formed by cerium ion and zirconium ion in the complex oxide, and about half of the pyrochlore phase-type ordered array phase remaining even after high-temperature heating in air (PTL 1, claim 1).